Dual reed comb filter



April 1969 H. 3. MILLER 3,440,534

DUAL REED COMB FILTER Filed Aug. 12, 1965 40c g 2 INVENTOR 3/ 0 Horace 6. Miller United States Patent 3,440,534 DUAL REED COMB FILTER Horace G. Miller, 4800 W. 55th Terrace, Mission, Kane. 66611 Filed Aug. 12, 1965, Ser. No. 479,265 Int. Ci. G012: 23/00 US. Cl. 324-80 1 Claim ABSTRACT OF THE DISCLOSURE A frequency indicator, particularly adapted for use by musicians, employs groups of vibratory reeds corresponding to the octaves of the frequency spectrum to be monitored by the device. Each group of reeds comprises a bank of vibratory members, each of the latter having a longitudinal opening therein extending from its free end to a point spaced from a clamp which provides cantilever support for the members. Each member, therefore, is split into a pair of vibratory reeds joined adjacent the clamp by an integral web which serves as a mechanical coupling for transmitting vibratory motion from one reed of the pair to the other. Only one reed of each pair is electromagnetically driven, thus the frequency response of the composite reed pair is affected by the integral web coupling, with the result that a relatively flat comb filter frequency response characteristic with sharp skirt selectivity is produced. A filter is coupled with the electromagnet associated with the lowest frequency reed group to prevent sec-0nd mode response when the fundamental frequency of the input signal is in the octave range of the third lowest frequency reed group.

This invention relates to frequency responsive devices of the band-pass type employing vibrating reeds as frequency responsive components and, more particularly, to a novel reed configuration which provides a passband response characteristic having improved skirt selectivity and adjacent channel signal attenuation.

Vibrating reed frequency indicators are used in a number of applications where it is desired to identify the frequency of an electrical signal, usually in the audio range. A particular application of indicators of this type is found in the musical arts where the indicator is employed as an instructional aid to assist in teaching an individual to sing a cappella. In use, the student views the indicator while singing into a microphone which is operably coupled therewith. In this manner, any tendency of the vocalist to sing sharp or flat with respect to the written notes is detected by the indicator so that the voice may gradually be trained to accurately follow the score.

Prior art indicators of this type, however, have a number of disadvantages. First, unless mechanical damping of the reeds is employed, the width of high amplitude response in each channel is quite narrow, necessitating that the vocalist be accurately on pitch before a high amplitude reed vibration is obtained. Secondly, at lower vibrational frequencies the band widths of the reeds broaden appreciably and often overlap adjacent semitones, thereby producing a multiple frequency indication. Thirdly, many prior art indicators are difficult to read, both because of a combination of the two adverse effects discussed above and the manner in which the reeds are dis posed for viewing and associated with the frequency calibrations or note indicia of the instrument scale.

It is, therefore, the primary object of this invention to provide a frequency responsive, vibrating reed bandpass device having a comb filter response characteristic which eliminates adjacent channel interference and provides a relatively constant reed vibration amplitude over each passband.

As a corollary to the foregoing object, it is an important aim of the instant invention to provide a vibrating reed frequency indicator which may be easily read by the viewer due to the elimination of multichannel response and the provision of improved means of identifying the frequencies of the reeds.

Another important object of this invention is to provide a frequency indicator having wide band widths and sharp skirt selectivity, thereby especially adapting the indicator for use as a vocalists instructional aid since the singer may deviate slightly from the exact frequency of a particular note and still obtain a high amplitude vibration from the appropriate reed.

Still another important object of this invention is to provide a vibrating reed. band-pass device in which the width of the passband may be readily set during fabrication of the instrument, and wherein mechanical damping of the reeds is not required in order to increase the band width.

A specific object of the instant invention is to provide a resonant, vibratory member of dual reed construction having a superior passband characteristic, and wherein the width of the passband is controlled by the degree of mechanical coupling between the reeds.

A further object of the invention is to provide a frequency indicator having an extended operating spectrum wherein means is employed to provide relatively constant reed deflection sensitivity.

An additional object of this invention is to provide a frequency indicator having an extended operating spectrum wherein means is employed to suppress harmonic mode reed response.

In the drawing:

FIGURE 1 is a front, elevational view of a frequency indicator made in accordance with the teachings of the instant invention, the front cover thereof being broken away to reveal details of construction;

FIG. 2 is a vertical sectional view taken along line 2-2. of FIG. 1;

FIG. 3 is an enlarged, details, side view of one of the one-octave indicator units;

FIG. 4 is a vertical sectional view taken along line 4-4 of FIG. 3;

FIG. 5 is a plan view of the unit shown in FIG. 3;

FIG. 6 is a diagrammatic illustration of the vibratory member made in accordance with the instant invention;

FIG. 7 is an electrical schematic diagram of the driver and filter circuit; and

FIG. 8 is a comparative graphic illustration of the passband response characteristic of the instant invention and the characteristic of prior art, single reed devices.

In brief, the instant invention comprises a band-pass device of the vibratory reed type having a dual reed arrangement which produces a relatively fiat-topped comb filter frequency response characteristics with sharp skirt selectivity. As shown in the drawing and described hereinafter, the device is useful as a frequency indicator and employs typically three groups of vibratory reed assemblies corresponding to a three octave frequency spectrum. Each octave group includes a bank of vibratory members supported by a clamp, each member having a longitudinal opening from its free end to a point spaced from the clamp to split the member into a pair of vibratory reeds joined adjacent the clamp by an integral web. One reed of each reed pair referred to hereinafter as the primary reed is disposed within an exciting magnetic field, while the other or secondary reed diverges from the primary reed and is disposed out of the field. Therefore, when one of the primary reeds is in resonance with the applied field, the vibratory motion thereof is transmitted to the secondary reed by the mechanical coupling provided by the integral web, the composite effect being to flatten the frequency response of the device at each resonant frequency and sharpen the skirt selectivity.

The numeral designates a case provided with a front cover 12 and mounted on a stand 14 by a pair of pivotal connections 16. A mounting strip 18 is disposed within case 10 and extends along the rear wall thereof, strip 18 serving to support three indicator assemblies 20, 22 and 24. An indicia bearing member 26 having an elongated, rectangular, calibrated face 28 is disposed above assemblies -24 and extends horizontally between the sidewalls of case 10. It may be noted that face 28 is calibrated to represent a musical scale covering a range of three octaves; the base clef notes G through Cit at the left end of face 28 (FIG. 1) being hidden by cover 12.

Except for the frequency range thereof, each of the assemblies 20-24 is similar in construction, a representative assembly being shown in detail in FIGS. 3-5. A nonmagnetic mounting block 30 has four bar magnets 32 cemented or secured to one face thereof by any suitable means, the arrangement of the magnets being clearly shown in FIG. 4. Three nonmagnetic spacers 34 are inserted between the magnets 32, the lower ends of the magnets engaging a magnetic shunt 36. Magnets 32 are arranged such that their lower ends are of the same polarity (and thus their upper ends are of the same but opposite polarity), shunt 36 thereby serving to intercouple these poles of the magnets with a magnetic mounting plate 38 which supports a transducer 40 in the form of an electromagnet provided with a core 42 having opposed magnetic poles 44 and 46. Pole 46 presents a pole face which is covered by a thin pad 48 and disposed in underlying relationship to a bank of primary, magnetic, vibratory reeds 50, and a bank of secondary, magnetic, vibratory reeds 52 (FIG. 3). The two reed banks 50 and 52 are held by a clamp in the form of a pair of magnetic plates 54 secured to block 30 by a pair of machine screws 56. The upper ends of magnets 32 abut the lower plate 54, thereby establishing a series magnetic circuit through magnets 32, plate 38, core 42, and reed blanks 50 and 52 to clamping plates 54.

The reed banks 50 and 52 are integrally formed and comprise twelve elongated members 58, each provided with a longitudinal opening communicating with the free end thereof. One of the members 58 is diagrammatically illustrated in FIG. 6, wherein it may be seen that the opening 60 elfectively splits member 58 into a pair of vibratory stretches 62 having free extremities 64 remote from clamping plates 54. A portion of the upper clamping plate 54 is broken away in FIG. 5 to reveal that the roots of members 58 are integral with a flat, thin strip 66 which is clamped between plates 54. In should also be noted that each member 58 is clamped above its root 67, and that an integral element 68 joins the clamped end portions of stretches 62 of each member 58 at the root 67 thereof, a segment of each element 68 extending free of clamping plates 54 (FIG. 6). Each segment 70 thereby provides zones of interconnection with its asso-.- ciated stretches 62 to provide a vibratory motion transmitting, mechanical coupling between the two stretches.

The two stretches 62 of each member 58 form primary and secondary vibratory reeds of substantially the same resonant frequency which are associated with the primary bank 50* and the secondary bank 52, respectively. Adjacent members 58 differ in frequency by one semitone of the tempered scale; thus, the twelve members 58 of each assembly 20, 22 or 24 cover a frequency range of one octave. The secondary bank 5'2 diverges from the primary bank 50 as extremities 64 are approached in order to reduce the magnetic coupling of the field of electromagnet 40 with secondary bank 52. The divergence is effected by bending stretch 62 forming the secondary reed of each member 58 away from the stretch 62 which forms the primary reed thereof, as is clear in FIGS. 3-5.

The resonant frequencies of the three groups of reeds of assemblies 20-24 increase in frequency progressively from left to right as viewed in FIG. 1. Therefore, a wedge 72 is disposed between the rear of each block 30 and mounting strip 18 so that the extremities 64 of the reeds will be aligned beneath face 28. Only the wedge 72 behind assembly 24 is visible in the drawing (FIG. 2).

An upright screen 74 (FIG. 2) is mounted in cover 12 and extends upwardly to a disposition between the banks of primary and secondary reeds to obscure the primary reeds from view through magnifying lens 76. It will be appreciated, therefore, that only the secondary reeds and the calibrated face 28 are visible to the viewer. The extremity 64 of each secondary reed is :coated with a specular substance 78 (FIG. 5) and illuminated by lamps 80. A black strip 82 is mounted behind the secondary reeds to provide a dark background. Substance 78 may take the form of white semigloss paint, thereby providing a sharp contrast against the black background of strip 82.

FIGURE 7 shows the manner in which the three electromagnets are connected to a pair of input terminals 84. The coil of the electromagnet of assembly 20 is designated 40a, the coils of the electromagnets of the higher frequency assemblies 22 and 24 being designated 40b and 40c respectively. It will be noted that the electromagnet coils are connected in a current divider network with coils 40a and 40b being connected in series across coil 40c. A resistor 86 in series with coil 40b and a resistor 88 in parallel with coil 40a serve to set the relative current magnitudes in coils 40a and 40b at predetermined, desired levels.

The purpose of the current divider network is to provide constant reed deflection sensitivity, since the amplitude of vibration of a vibratory reed is approximately inversely proportional to the square of its resonant frequency. Thus, in the three octave arrangement illustrated, the values of resistors 86 and 88 are such that the instantaneous magnitudes of the currents in coils 40a, 40b and 400 differ by a factor of four, i.e., the current through coil 40a is A of the current through coil 40b, and the current in coil 40!) is A of the current in coil 40c.

Capacitor 90 in parallel with resistor 88 and coil 40a is of a value to prevent second mode response (6.25 times the fundamental frequency) of the reed group associated with coil 40a when the fundamental frequency of the input signal is in the octave range of the reed group associated with coil 40c. Capacitor 90 effectively shunts highor frequency signals around coil 40a and thus provides a simple low-pass filter to prevent coil 40a from receiving a substantial amount of current drive except when the applied signal is in the octave range of the reed group of assembly 20.

FIGURE 8 shows the passband response characteristic of three vibratory members 58 having center frequencies separated by an interval of one semitone. The center frequencies are illustrated at 92, 94 and 96 along the frequency axis of the graph. The graph ordinate represents the amplitude of vibration of the vibratory members, it being apparent that each member exihibits a double peak characteristic as shown at 98. Although there is a slight reduction in amplitude between each pair of peaks 98, the response is substantially flat between the peaks and then decreases sharply to present skirts 100. Thus, a combfilter characteristic is exhibited by the vibratory members,

adjacent skirts 100 intersecting at approximately onehalf of the maximum amplitude of vibration.

A second set of three characteristic curves are designated 102 and illustrate the response normally obtained by three partially damped, single reeds having center frequencies at 92, 94 and 96. Assuming a signal having a frequency equal to center frequency '94 is applied to the three single reeds, it is evident that all three reeds will respond because of the substantial overlap of curves 102. Additionally, a high magnitude of reed vibration will not be obtained unless the frequency of the exciting signal is exactly the same as the center frequency 94, or very close to such frequency, due to the sharp peaked characteristic produced by each of the single reeds.

The passband response characteristic of the vibratory members '58, however, suffers from neither of these disadvantages since sharp skirt selectivity is achieved and the passband between the peaks 98 is relatively flat.

In the utilization of the instant invention as a vocalists training aid, the three octave spectrum covered by the three reed groups of assemblies 2024 encompasses the usual singing voice ranges. The note C in the center of face 28 corresponds to middle C on the piano keyboatrd and is aligned with the sixth secondary reed from the left end of assembly 22, as viewed in FIG. 1. The spectrum of the device thus extends from the G below low C to the Fit above high C. This extended spectrum covers the normal voice ranges of most individuals including male basses and female sopranos.

Terminals 84 may be connected to any suitable signal source such as, for example, the output transformer of an audio amplifier which would normally be utilized to drive a loud speaker. The frequency of the incoming signal is indicated by the three secondary reed banks 52, the extremities 64 of the reeds thereof being disposed directly below corresponding note indicia on face 28. As set forth above, the three assemblies 20-24 are mounted at an angle with respect to mounting strip 18 so that the extremities of the secondary reed banks are in linear alignment; FIG. 1 clearly reveals that the face 28 of member 26 extends directly over the secondary reed banks 52 in parallelism with the aligned extremities.

The three electromagnet coils 40a, 40b and 400 are excited by the incoming signal and cause magnetic fields to be produced which are coupled with the three primary reed banks 50. These fields fluctuate in intensity in accordance with the frequency of the incoming signal, and add a variable component to an existing fixed magnetic field produced by the various bar magnets 32. In this manner, the reeds are excited by a varying magnetic field, and the reed pair having a resonant frequency equal to the frequency of the incoming signal is caused to vibrate. The fixed component of the field serves to maintain a constant magnetic flux which prevents distortion and spurious responses by maintaining the magnetic force on the reeds linearly related to the current flowing in the electromagnetic coils.

Since the secondary reed banks 52 are purposely disposed out of substantial coupled relationship with the magnetic fields, it will be appreciated that vibration of the secondary reeds to any substantial degree would not be efiected without some means of coupling the driven, primary reeds with respective secondary reeds. This is accomplished since the element 68 integral with each pair of vibratory reeds or stretches 62. serves as a stiffness coupling to transmit vibrational energy from the primary reed to the secondary reed. This is especially clear in FIG. 6 where it may be seen that a segment 70 of the stiffness coupling element extends free of clamping plates 54 and forms an integral web interconnecting the two vibratory stretches 62. The coupling coefiicient is approximately determined by the relationship A/L, where A represents the length of segment "70 and L represents the length of each of the stretches 62, as illustrated in FIG. 6.

The relationship A/L is also approximately equal to the band width of a particular vibratory member 58 divided by the center frequency of its passband. Thus, as the length of segment 70 is increased, the band Width of the member 58 increases and the peaks 98 of its response characteristic become more widely separated. These relationships assume that both the primary and secondary reeds or stretches 62 are tuned to the same frequency and that the secondary reed is spaced at sufficient distance from the electromagnet to preclude substantial magnetic coupling therewith. In practice, a reduction of the magnetic driving force on the secondary reed by a factor of approximately two as compared with the magnetic driving force applied to its associated primary reed is sufiicient to enable the apparatus to produce the passband characteristic illustrated in FIG. 8 and described hereinabove.

A coupling coefficient of approximately 0.06- is utilized in the instant invention when the same is employed as a voice frequency indicator. This is because of the relationship between the various semitones of the tempered scale which requires a constant percent band width for each note throughout the frequency spectrum of the instrument. This provides the particular characteristic illustrated in FIG. 8 where the skirts 100 of adjacent passbands intersect at approximately the one-half amplitude point.

It will be appreciated, however, that in applications where constant absolute band width for each reed pair is required, such as in a frequency analyzer where face 28 would be calibrated numerically, the coupling coefficient would constantly decrease as the center frequency of each reed pair increases. Furthermore, the band width may be selected to be of a greater or lesser breadth than illustrated in FIG. 8, depending on the response characteristic that is desired. Increasing or decreasing the stiffness coupling to vary the band width does not substantially alter the curve selectivity but, instead, shifts the peaks 98 toward and away from one another.

In fabrication of the device, the stiffness coupling is easily adjusted through the use of clamping plates 54 until the desired length A (FIG. 6) is obtained. Each pair of primary and secondary reeds is tuned to the center frequency of its passband. However, a slight detuning of the primary reeds is an expedient which may be utilized to achieve the desired passband curve shape after the clamp is set. This is accomplished by securing the clamp by screws 56 with the lengths A slightly less than required to produce the desired passband width. Then, the free extremities of the primary reeds are loaded by the application of solder or lacquer thereto to cause an increase in the distance between peaks 98 until the desired bandwidth is obtained.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:

1. In a frequency indicator:

a plurality of groups of magnetic, vibratory reeds having a spectrum of resonant frequencies,

said groups being at least three in number and having respective one octave frequency ranges encompassing successively higher frequency portions of said spectrum;

a plurality of electromagnets for producing magnetic fields coupled with respective groups;

terminal means for receiving electrical input signals;

and

circuit means coupling said electromagnets with said terminal means for applying said signals to the electromagnets,

said circuit means including a network for dividing current flow through said electromagnets in accordance with the frequency ranges of their associated reed groups to provide current magnitudes in the electromagnets approximately proportional to the square of the frequencies of corresponding octave notes, and further including filter means operably coupled with the electromagnet associated with the lowest frequency reed group for preventing second mode response thereof when the fundamental frequency of the input signal is in the octave range of the third lowest frequency reed group.

References Cited UNITED STATES PATENTS Lingel 324-80 Dyner 3248O X Daschke 324 0 10 Pleasure 324-80 X Balamuth 84-454 X 8 FOREIGN PATENTS 8/ 1909 Germany.

OTHER REFERENCES Electronic Industries, August 1944, Frequency Range Extension.

RUDOLPH V. ROLINEC, Primary Examiner.

P. F. WILLE, Assistant Examiner.

US. Cl. X.R. 84454 

